Extrusion method

ABSTRACT

A diverter and coextrusion diverter are described which include means for adjusting the cavity of the diverter so as to control the uniformity of extrudate exiting the diverter.

This is a division of application Ser. No. 413,301 filed Aug. 31, 1982,now U.S. Pat. No. 4,495,022.

TECHNICAL FIELD

This invention relates to methods for forming uniform extrudates andparticularly to methods for extruding tubular jackets about cables andthe like.

BACKGROUND OF THE INVENTION

While the present invention is useful generally wherever uniformextrusion of an extrudate onto a substrate is desired, the inventionwill be discussed primarily with regard to extruding tubular jacketsabout cables such as those used in the telecommunications industry.

In the manufacture of telecommunications cable a protective plasticjacket is often extruded over a stranded cable core. This is generallydone with an extruder of the type having a crosshead. A crosshead servesto redirect and to reconfigure a stream of plastic fluid, usuallycylindrically or rod shaped, into a tubular shape about the cable. Thisreconfiguration is generally performed with the use of a diverter tubethat is seated within a cylindrical cavity of a crosshead block. Theinterior surface of the diverter tube is sized to receive the cablewhich is passed linearly therethrough, or to receive a core tube throughwhich the cable is passed, while its generally cylindrical exteriorsurface is provided with raised lands that define channels. Thesechannels are shaped so as to redirect the flow of plastic fluidintroduced into the crosshead some 90° and to divide it into two or morestreams that are routed to a plurality of diverter tube channel passagesspaced radially along the cylindrical cavity. From here theconfiguration of the raised lands is such as to permit the plastic fluidto spread into a tubular confluent that is drawn down upon the cablecore just outside the extruder die.

As the fluidic plastic material must ordinarily follow a flow pathhaving changes in both direction and path size, varying flow and balanceconditions are inherently created. These flow and balances, whereinvarious portions of the flow at any one point along the path travel atdifferent speeds, create circumferential variations in the thickness ofthe wall of the tubular confluent once it has solidified into a jacket.Since some minimum wall thickness is required for proper cableperformance, these circumferential variations in jacket thickness mustbe compensated for by an increase in the average wall thickness. This,of course, increases manufacturing costs.

The just described problem of wall thickness variations in extruded,tubular jackets has heretofor been recognized and attempts made atproviding solutions. These solutions have taken the form of crossheaddesigns that divide the stream of plastic fluid delivered to thecrosshead into several smaller branch streams that are routedcircumferentially about the cable and then recombined into a tubularconfluent stream to equalize the flow rate of plastic circumferentiallyonto the cable. This has been done on a volumetric or flow rate basis,neglecting pressure and velocity distributions in the plastic stream.Though such designs have improved concentricity and roundness of tubularextrusions, they have not been satisfactory when the plastic utilizedhas been of a highly viscous or elastic type. Molten polypropylene ormedium density polyethylene, for example, when moving through a conduitis subjected to shear stresses that result in substantial velocity andpressure gradients, particularly in channel bends and enlargements whichdo not readily return to steady state fluid flow conditions.

Two examples of the just described approaches are shown in U.S. Pat.Nos. 3,579,731 and 3,860,686. These patents disclose a crosshead havinga compensation or diverter tube formed with an annular restrictionlocated downstream of a fluid delivery port. The restriction has anaxial length that tapers from an axially long surface located radiallyadjacent the delivery port to an axially short surface located radiallyopposite the delivery port. This construction has been found to performwell where the taper is designed for specific flow rate of a plasticfluid of known viscosity. Its effectiveness, however, is diminishedsignifically when plastic fluids of other viscosities are used or whereother flow rates are employed. Consequently, these approaches are notreadily adaptable to changes in processing material, tooling andextrusion conditions. Furthermore, such prior art jacketing divertertubes are difficult to fabricate due to their design complexity whichleads to increased fabrication costs. In addition, use has been limitedto the application of extruded insulation about wires as opposed tocables. It would be quite difficult to use this prior art apparatus inthe manufacture of cables due to the size limitation which flow andpressure requirements impose on cable jacketing extruders.

Another problem resulting from existing cable jacketing diverter tubesis the precise alignment which must be maintained between the divertertube and the incoming plastic flow stream. Also, weld lines formed onthe tubular jacket are often weak.

More recently, an improved diverter tube has been described in U.S. Pat.No. 4,279,851. Here, a stream of plastic fluid is forced into thecrosshead and the stream is bifurcated into two branch stream. Eachbranch stream is channeled into two diametrically opposed locationsabout the cable where each stream is shaped with the selected prespreadwidth. Each stream is spread from each of these locations into aconfluent stream about the cable of tubular shape having a circumferenceof between 22 and 50 times the prespread width of each branch stream atan axial to lateral spread ratio of between 1:2 tangent 30° and 1:2tangent 40°.

While this latter design is less complex and less costly than the formermentioned apparatus, there still exists the problem of the formation ofweak weld lines and the lack of adaptability to rheological changes dueto changes in processing material, tooling and extrusion conditions aswell as a desire for further diverter cost reduction.

It can therefore be seen that need still remains for the development ofpractical and cost effective methods and means for extruding plasticjackets of uniform tubular wall thickness about cables and the like,especially, for methods and means which are adaptable to changes inprocessing materials, tooling and extrusion conditions.

SUMMARY OF THE INVENTION

A method of jacketing a cable or other workpiece comprises: (1) drawingthe workpiece through a diverter having a circumferentially and axiallyextending cavity therein said cavity including an adjustable contractiongap through which extrudate must pass before being shaped and sized in adie section of the cavity; (2) forcing extrudate fluid into the divertercavity through an extrudate entry port in the diverter whichcommunicates with a section of the cavity prior to the adjustablecontraction gap; (3) allowing extrudate to flow out of an annularextrudate exit orifice and to be drawn down over the workpiece as it isdrawn through the diverter; and (4) adjusting the size of thecontraction gap until the desired uniformity of extrudate jacket overthe workpiece is achieved for the given extrudate and operatingparameters.

The invention further includes the method of jacketing a workpiece,e.g., a cable, advanced through the diverter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway isometric elevational view of a noveldiverter-die assembly;

FIG. 2 is a cross-sectional view of the top half of the diverter-dieshown in FIG. 1; and

FIG. 3 is a cross-sectional view of a coextrusion diverter in accordancewith the invention.

DETAILED DESCRIPTION

A diverter generally functions to convert a rod type extrudate into atubular shape. This tubular shaped extrudate may then be further sizedthrough a separate die coupled to the outlet of the diverter or througha die which is an integral part of the diverter. Generally, tubing diesare characterized by relatively small pressure drops along the diverterpath as compared with a pressure die which generally has a high pressuredrop within the die. The novel diverter, however, can operate in eithermanner depending upon the nature and fit of the novel diverters centralcore member as will be more fully explained hereinafter.

Generally, the novel diverter comprises first and second cylindricalmembers having means for adjustably coupling one member with the other.The members have a hollow core through which either a core tube which iscapable of passing a workpiece, e.g., a cable, is positioned or throughwhich the workpiece is directly positioned without a separate core tube.When a core tube is employed, as is generally preferred, the tube isadjustably positioned with respect to the other members forming the dieso as to provide an annular extrudate outlet. In this configuration thecable or other workpiece to be jacketed is passed through the center ofthe core tube. The size of the cable or other rod shaped workpiece whichcan be accommodated by the diverter can be altered within limits for agiven core tube or one can simply replace the core tube with anothercore tube of different inner diameter in order to accommodate differentsize cables or the like. When operating in this manner the diverterfunctions as a tubing die and the cable becomes jacketed with extrudateoutside of the confines of the die. Since cable is generally corrugatedand does not have a uniform cross section, the tubing die configurationis preferred for forming jackets thereover.

When no core tube is present the workpiece to be jacketed becomes thecentral core and must fit snugly in the central axial cavity of thediverter such that extrudate cannot be forced backwards through thediverter but yet the fit must allow the workpiece to be freely drawnthrough the diverter. When operating in this manner the diverterfunctions as a pressure die and the workpiece becomes jacketed withinthe confines of the die.

Referring now to FIGS. 1 and 2, there is shown a diverter-die suitablefor use as a tubing die for jacketing a cable or other workpiece whichis passed therethrough.

The diverter, as shown, comprises the part housing 10. The parts of thehousing are a cylindrical outer member 12 having an essentially`E`-shaped cross section, a cylindrical axially extending front member14 which is adjustably coupled to the outer member 12 by means of screwthreads 16 and 18 provided on the inner surface of the front portion ofthe outer member 12 and the outer surface of the rear portion of thefront member 14, respectively, and a central core tube 20 member whichextends axially through the center of the outer member 12 and frontmember 14 and is spaced from the inner surface of front member 14 and isadjustably coupled to the outer member 12 by means of a screw threads 22provided therefor.

The outer member 12 is provided with an extrudate entry port 24 whichaccepts the extrudate from an extruder for transfer into an internalcavity 26 formed by spaces between the various housing members.

The cavity 26 comprises three sections: a ring section 28 whichcommunicates with the extrudate entry port 24, a final or die section 30which extends circumferentially and axially along the top surface 32 ofthe central core member 20 and terminates in an annular extrudate outletorifice 34 formed by a space between the central core member 20 and thefront of the front member 14, and an narrow contraction section 36between and communicating with the ring and die sections 28 and 30. Thecontraction section is formed by an adjustable gap existing betweenopposing internal faces 38 and 40 of the first and second members 12 and14, respectively.

It should be understood that the various members can be adjustablycoupled by means other than screwing one into the other as is well knownin the art. Further, it should be understood that while the centralmember is shown here as a core tube, this tube can be replaced by theworkpiece to be jacketed, in which case no screw threads would beprovided around the central hole of the outer member and the workpiecewould be of a size slidably but snugly fit in such hole therebyconverting the tubing die shown into a pressure die.

As can be seen from FIG. 2 in the configuration shown, a cable 42 ispassed through the core tube 20 and is coated with extrudate 44 outsidethe confines of the diverter. The front portion of the core tube 20 ispreferably provided with an inwardly extending flange 46 which acts as aguide to keep the cable 42 centralized.

The screw adjustment which couples the outer and front members 12 and 14is used to adjust the size of the gap forming the contraction section 36of the diverter. It is this adjustment that balances and controls theuniformity of extrudate flow and hence the uniformity of the jacketformed over the cable. Further, it allows one to utilize the samediverter for varying extrudate rheologies resulting from varying theextrudate material or operating conditions. The adjustment of the coretube 20 allows one to align the front end of the core tube 20 with thefront face of the front member 14 so as to control the extrudate leavingthe diverter and the point at which it contacts the cable so as toobtain a smooth bubble free jacket over the cable of the properdimension.

While the diverter as shown includes an annular exit port which is thefinal desired size of the jacketing material, it should be understoodthat the diverter could have terminated in a die section which is widerthan the final desired jacket thickness and a separate sizing die wouldthen be coupled to the exit port of the diverter so as to obtain thefinal desired jacket thickness of the extrudate.

In the preferred embodiment of the invention, in order to achievemaximum uniformity of extrudate flow when the extrudate leaves thecontraction section 36 of the diverter cavity 26 the gap forming thecontraction section should be non-uniform, the gap being larger 180°from the vicinity of the entry port as compared to the vicinity of theentry port and preferably should be smallest near the entry port 24 andlargest 180° from the entry port 24. This can be achieved by bevelingone or both faces 38 and 40 forming the gap. Since the face 38 on theouter member 12 is stationary with respect to the entry port 24 it isbest to bevel this face. If however, the front member 14 was coupled tothe outer member by other than a rotary motion as in the screw coupling(e.g., by providing a keyed, slide fit with a set screw to lock it inany desired position) then beveling the face 40 of the front member 14would be equally suitable. The angle between the opposing faces shouldgenerally be between about 1/2° to 5° and is usually preferably between1/2° and 3° depending upon the size of the diverter and other factors.Improved results are generally obtainable with gap angles in this range.It should be noted that it has been determined that the actual optimumvariation in gap size from entry port to 180° from the entry port isactually a non-linear change in gap size. However, the cost of providingsuch a non linear variation in gap size as opposed to the calculatedadded advantage over a linear change is not believed to be generallyworthwhile. Generally, for purposes of comparison commonly employedspider dies (multi-channel dies) creates a plurality of weld lines wherethe flows from each channel join. These weld lines are generallyvisually perceptible and are often sources of failure of the jacketedcable due to weakness of the jacket at the weld line. By employing thenovel die of this invention, there is only one region where fluid flowjoins (180° circumferentially from the extrudate entry port) and sincethis occurs in the first stage of the diverter cavity, i.e., the ringsection, the weld lines generally become imperceptible and there islittle or no reduction in material strength along this line.

Referring to FIG. 3 there is shown a novel diverter useful forcoextrusion so as to form a dual layer jacket. This diverter 100 may bedescribed as a two stage, tandem diverter and is comprised of thefollowing parts: a main cylindrical housing 102; a first stagecylindrical constriction gap forming member 104; a first stagecylindrical cover plate 106; a second stage cylindrical constriction gapforming member 108; a front die sizing member 110; and a core tube 112.

The housing 102 is provided with first stage and second stage extrudateentry ports 114 and 116, respectively. These entry ports 114 and 116communicate with respective first and second stage ring sections 118 and120 of the first stage and second stage cavities 122 and 124. An annularexit 126 of the first stage cavity 122 communicates with the base 128 ofthe die section 130 of the second stage cavity 124, at the rear mostportion of the die section 130. The cavities 122 and 124 which extendcircumferentially around and within the diverter 100 are formed by meansof the combination of the contours of the housing 102 and the relativepositions of the other members (104, 108, 110 and 112) within thehousing 102.

More specifically, the contraction gap forming member 104 is adjustablymounted to the rear inner portion of the housing 102 by means of a screwthread provided therefor. The member 104 is cylindrical and has a rearoutwardly extending flange portion 132 which lies within a rear flangeportion 134 of the housing 102. The flange 132 is provided for ease ofadjustment of the position of the member 104. Once adjusted, thecylindrical cover plate 106 is mounted to the housing 102 by means of aplurality of mounting bolts 136. The position of the member 104 withinthe housing 102 defines the first stage cavity ring section 118 andprovides the adjustable gap between its front face 138 and an opposinginternal face of the housing 140 which forms the first stage cavitycontraction section 141. The core tube 112 which is shown to consist ofa rear section 142 and a front section 144 screwed together at thebeginning of the second stage cavity die section 130 extends axiallythrough a central core of the housing 102 and abuts the innercylindrical surface of the first stage contraction gap forming member104. it is movable within the housing 102 by means of a screw coupling146 to the inner surface of the cover plate 106. The core tube 112 isspaced from the central portion 148 of the housing 102 which space asextended back to the front face 138 of the first stage contractionforming member 104 forms the die section 150 of the first stage cavity122.

The ring section 120 and the contraction section 152 of the second stagecavity 124 is formed by a space between the housing 102 and the rearface of the second stage contraction gap forming member 108. The diesection 130 of the second stage cavity is formed by a space between thecore tube 112 and the second stage contraction forming member 108 andthe front sizing member 110 which surrounds the front section 144 of thecore tube 112. The front sizing member 110 which is positioned withinthe member 108 so that in operation, the pressure causes them to bejuxtaposed and constrained by mating lips on the two members. The gapsize of the contraction section 152 of the second stage is varied byadjusting the screw position of the contraction forming member 108 whichcouples this member to the front portion of the housing 102. As can beseen from the figure the size of the contraction section gap is larger180° from the extrudate entry ports as compared with the contraction gapsize closest to the entry ports.

In operation, extrudate, e.g., a first polymer having a first viscosityis forced into the first stage entry port 114 where it enters the ringsection 118 of the cavity 122 and distributes around the ring section toform a solid ring. The extrudate is then forced through the contractionsection of the cavity in a balanced flow from around the ring sectionand thence into the die section of the first stage. The first polymerexits the annular exit of the first stage die section and enters thesecond stage die section. Extrudate, e.g., a second polymer having aviscosity the same as or different from the first polymer is then forcedinto the second stage entry port. This extrudate distributes throughoutthe ring section and contraction section of the second stage cavity andenters the die section of the second stage cavity where it overlies theextrudate of the first polymer therein. A coextruded, laminar, tubularshaped extrudate is thus forced out of the diverter. This coextrudedmaterial then is drawn down around the cable or other workpiece which ispassed through the central cavity of the core tube.

EXAMPLE I

Employing a diverter as described with reference to FIGS. 1 and 2wherein the width of the cavity ring section is 1.75 inches; the heightof the ring section is 0.61 inches; the mean radius of the ring section(the distance from the center of the diverter outwardly to the center ofthe ring section) is 1.97 inches; the mean radius of the contractionsection (the distance from the center of the diverter outwardly to thecenter of the contraction section) is 1.5 inches; and the length of thecontraction section is 0.3125 inches for the jacketing of a 11/2 inchdiameter cable with Union Carbide 6059 linear low density polyethylenemaintained at a temperature of 440° F., under a head pressure of lessthan 1,000 psi and at a flow rate of 120 lbs./hr. The followingrelationship between gap divergence angle, gap size and meancircumferential pressure deviation has been theoretically calculated.The zero shear rate viscosity of this polymer is 3.312 psi-sec^(n)wherein n is the Power-Law Index (which in this case is 0.347). Also thepressure drop in the die section of the cavity is about 350 psi, asobtained in a separate calculation.

The mean circumferential pressure deviation is directly related to theuniformity of the extrudate jacket formed on the cable, i.e., thecircumferential deviation in thickness along the cable.

The following table therefore sets forth the various deviations injacket thickness as a function of contraction gap size and gapdivergence angle. The table also shows the pressure drop in thecontraction section of the diverter cavity in psi.

    ______________________________________                                                                         Mean                                         Gap Divergence          Pressure Pressure                                     Angle        Gap Size   Drop     Deviation                                    (degrees)    (inches)   (psi)    (percent)                                    ______________________________________                                        0.00         .050       614.12   6.6%                                         0.00         .075       482.89   8.1%                                         0.00         .100       431.63   9.0%                                         0.00         .125       405.94   9.4%                                         0.00         .150       391.07   9.7%                                          .25         .050       614.12   1.1%                                          .25         .075       482.89   5.3%                                          .25         .100       431.63   7.5%                                          .25         .125       405.94   8.6%                                          .25         .150       391.07   9.2%                                          .50         .050       614.12   5.8%                                          .50         .075       482.89   3.0%                                          .50         .100       431.63   6.3%                                          .50         .125       405.94   7.9%                                          .50         .150       391.07   8.7%                                          .75         .050       614.12   9.8%                                          .75         .075       482.89   1.2%                                          .75         .100       431.63   5.3%                                          .75         .125       405.94   7.2%                                          .75         .150       391.07   8.3%                                         1.00         .050       614.12   13.2%                                        1.00         .075       482.89    .9%                                         1.00         .100       431.63   4.5%                                         1.00         .125       405.94   6.7%                                         1.00         .150       391.07   7.9%                                         1.25         .050       614.12   15.9%                                        1.25         .075       482.89   1.9%                                         1.25         .100       431.63   3.7%                                         1.25         .125       405.94   6.3%                                         1.25         .150       391.07   7.6%                                         1.50         .050       614.12   18.2%                                        1.50         .075       482.89   2.9%                                         1.50         .100       431.63   3.1%                                         1.50         .125       405.94   5.8%                                         1.50         .150       391.07   7.3%                                         1.75         .050       614.12   20.2%                                        1.75         .075       482.89   3.8%                                         1.75         .100       431.63   2.5%                                         1.75         .125       405.94   5.5%                                         1.75         .150       391.07   7.1%                                         2.00         .050       614.12   21.9%                                        2.00         .075       482.89   4.6%                                         2.00         .100       431.63   2.0%                                         2.00         .125       405.94   5.2%                                         2.00         .150       391.07   6.8%                                         2.25         .050       614.12   23.4%                                        2.25         .075       482.89   5.3%                                         2.25         .100       431.63   1.6%                                         2.25         .125       405.94   4.9%                                         2.25         .150       391.07   6.6%                                         2.50         .050       614.12   24.7%                                        2.50         .075       482.89   5.9%                                         2.50         .100       431.63   1.2%                                         2.50         .125       405.94   4.6%                                         2.50         .150       391.07   6.4%                                         2.75         .050       614.12   25.8%                                        2.75         .075       482.89   6.5%                                         2.75         .100       431.63   1.0%                                         2.75         .125       405.94   4.3%                                         2.75         .150       391.07   6.3%                                         3.00         .050       614.12   26.8%                                        3.00         .075       482.89   7.0%                                         3.00         .100       431.63   1.0%                                         3.00         .125       405.94   4.1%                                         3.00         .150       391.07   6.1%                                         ______________________________________                                    

It can be seen from this table that there is an optimum gap size for anygiven gap angle and fixed extrudate material and operating parameters.However, the optimum gap size and/or gap angle will change with a changein extrudate material rheology. Since the novel diverter has anadjustable gap size, the optimum gap can be either recalculated or foundempirically by varying the gap size during operation of the die whileobserving the uniformity of the jacket.

The table indicates that for the particular die parameters andparticular extrudate conditions employed a minimum mean deviation isachieved at a gap divergence angle of 1.00° and a gap size of 0.075inches. Here, the mean deviation is only 0.9%. While small deviationsare also achievable at divergence angles of about 1/4°, e.g., 1/4° angle0.050 inch gap gives a 1.1% deviation, the pressure drop in thecontraction section under these parameters is greater than thatgenerally considered as desirable. Under the above conditions, thepressure drop was 614.12 psi. An arbitrary maximum generally set is 500psi.

It is to be understood that the above-described embodiments are simplyillustrative of the principles of the invention. Various othermodifications and changes may be devised by those skilled in the artwhich will embody the principles of the invention and fall within thespirit and scope thereof.

What is claimed is:
 1. A method of jacketing a cable or other workpiececomprises:(1) drawing the workpiece through a diverter having acircumferentially and axially extending cavity therein said cavityincluding an adjustable contraction gap through which extrudate mustpass before being shaped and sized in a die section of the cavity, andwherein the gap is nonuniform, there being a divergence of 1/2° to 5° ofmembers forming the gap such that the gap is smallest proximate theentry port for the extrudate and largest 180° circumferentially from theentry port; (2) forcing extrudate fluid into the diverter cavity throughan extrudate entry port in the diverter which communicates with asection of the cavity prior to the adjustable contraction gap; (3)allowing extrudate to flow out of an annular extrudate exit orifice andto be drawn down over the workpiece as it is drawn through the diverter;and (4) adjusting the size of the contraction gap until desireduniformity of extrudate jacket over the workpiece is achieved for thegiven extrudate and operating parameters.
 2. The method recited in claim1 including the step of continuing to draw and jacket the workpieceafter adjusting the gap to the desired size.